Fuel cell power plants are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus. In such power plants, one or typically a plurality, of planar fuel cells are arranged in a fuel cell stack, or cell stack assembly (CSA). Each cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. The reducing fluid and the oxidant are typically delivered to and removed from the cell stack via respective manifolds. In a cell using a proton exchange membrane (PEM) as the electrolyte, the hydrogen electrochemically reacts at a catalyst surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes, depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is a PEM electrolyte, which consists of a solid polymer well known in the art. Other common electrolytes used in fuel cells include phosphoric acid, sulfuric acid, or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials, is fixed and cannot be leached from the cell, and has a relatively stable capacity for water retention.
In operation of PEM fuel cells, it is usually desirable that a proper water balance be maintained between the rate at which water is produced at the cathode electrode including water resulting from proton drag through the PEM electrolyte and the rate at which water is removed from the cathode and anode electrodes. This is to prevent excessive drying or flooding of one or more of the various elements of the fuel cell.
In addition to water balance in the fuel cell power plant, there is the further requirement of a coolant system for maintaining appropriate temperature of the various components of the power plant. Typically, though not necessarily, the coolant will also be the water discussed above with respect to the need for water balance. The coolant is typically used to remove heat from certain portions of the fuel cell power plant, as for instance the fuel cell stack assembly (CSA), though the coolant may in some instances serve also as a source of heat. The coolant may also serve as a source of moisture for the control of humidification of various gas streams in the fuel cell power plant. In these ways the coolant serves to address the various heat loads of various portions of the fuel cell power plant.
The CSA may include a coolant plate means, or the like, that defines a coolant channel through the cell stack assembly, typically adjacent to the cathode, and which forms part of a coolant loop that is both internal and external to the CSA. The coolant loop typically includes at least a circulation means, such as a pump, and some form of heat removal means, such as a radiator. Inasmuch as the electrochemical reaction in the CSA may be the source of considerable heat, the coolant serves the important role of removing heat from the CSA. Coolant entering the CSA adjacent to the exiting cathode exhaust serves to cool the exhaust stream and condense water out of that gas stream, through the use of fine pore media such as the coolant plate means that define the coolant channel adjacent the cathode exhaust. The amount of heat removed is a function of the coolant temperature and flow rate of the coolant entering the CSA.
Because the coolant is recirculated in the coolant loop, the heat removal means performs the important function of removing, prior to its reintroduction to the CSA, most of the heat acquired during the coolant's passage through the CSA. While the heat removal means might take a variety of forms, by far the most common is that of an air-cooled radiator. Typically, it is the task of the radiator to remove all of the heat acquired by the coolant's passage through the CSA. The air which cools the radiator is typically at some ambient temperature associated with the environment of the fuel cell power plant, and may typically be, or approach, 120° F. (49° C.), particularly if the CSA is being used in a hot environment such as a desert. Because the temperature of the coolant exiting the CSA is not substantially greater than that of the radiator-cooling air, or stated conversely, because the temperature of the radiator-cooling air may be only a little less than that of the coolant exiting the CSA, the resulting relatively small temperature differential, sometimes referred to as the “pinch”, requires that the capacity of the radiator be relatively large in order to achieve the necessary cooling. On the other hand, this relative largeness of the radiator may be objectionable for several reasons, including initial cost, weight, size, appearance, and costs associated with its operation and maintenance.
Thus it is desirable to provide a fuel cell power plant in which the heat is managed in a manner allowing for a relative reduction in the sizing of the heat removal means, such as a radiator.